discussion Stirling Hot Air Engine

[Mr Bowcat]

Morning all.

Another model engine, this time an external combustion heat engine. Quite interesting as to the way it works by using the heating and cooling of air in an sealed environment to create oscillation.

If anyone would like to look at the plans in more detail they are freely downloadable from https://journeymans-workshop.uk/images/models/hotair/gammahae.pdf

The stock for this project was around £50, so fairly comparable to a decent plastic kit.

 

[PaulTRose]

This looks very intetesting

 

[Jim R]

Definitely interesting. Is the material you have copper, aluminium or what? Sounds reasonably priced.

 

[papa 695]

Very interesting Bob, is it a working model? I’ve just took a seat at the front for this.

 

[Mr Bowcat]

Jim, a mix of cast iron, steel, aluminium and brass. I realised when placing the order that a lot of it had a minimum spend, so by playing with the lengths I was able to get far more than I will need for the same price. Always good to have extra in case I screw a part up.

Ian, yes it should work when it's completed, assuming I build it correctly.

 

[minitnkr]

......verrrry interesting......

 

[Mr Bowcat]

Morning all.

With Storm Darragh raging all weekend I decided the safest place to be was the workshop.

I've been studying the plans and chose to tackle the displacer cylinder first, mainly because it involved silver soldering which I have never done before.

It started with some 16.5mm OD seamless steel tube which was turned at each end to square up and to get the correct length. This then had a piece of 1mm steel plate silver soldered to one end.

After turning off the excess material on the lathe I then made the flange for the other end. A piece of 2.5mm steel plate was squared up and drilled for the holes for mounting to the block later on. I also dropped in a center hole. The piece was then mounted in the lathe and the centre hole employed to get in concentric. Once done the hole was drilled to 14 mm, then a boring bar was used to open up the hole to just over 16.5mm. It needed to be slightly larger than the tube to allow somewhere for the solder to flow.To ensure it was the right size I bought a set of snap gauges (OK, it was just a good excuse to buy some). This was then soldered on and after some clean up the part was complete. You can see in the last pic the lighter coloured ring, which is where the solder went.

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I was really pleased with the silver soldering, it's not the prettiest, but as a first attempt went well, and more importantly the joints are strong and air tight.

With the cylinder out of the way I went on to make the displacer. This is a relativity simple part, just a length of 13.5mm OD ali tube. Each end was rebated and a plug of aluminium pressed in and turned down flush. Because this part will be at the hot side of the engine the plugs couldn't be chemically fixed with glue/threadlock, so they had to be made to an interference fit (a couple of thousandths of a mm larger than the inner diameter of the tube) and pressed in with my vice. The longer plug has a hole drilled in and tapped for M4.

Next up was the displacer rod, a length of 5mm drill rod, threaded one end M4 to screw into the displacer, and a 1.2mm hole and 2mm slot in the other end to eventually connect to the crank via a yoke. Another fairly simple part, though did involve using the murder saw (slitting saw) which frankly terrifies me every time I use it.

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Last part to be made was a pin for connecting the displacer rod to it's yoke. I actually made two in the same set up as another will be needed later down the line. These were tricky as they are only 1.2mm diameter, and were turned down from a 3mm stainless steel pin.

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And that's the displacer assemblies complete. The tube is the part that is heated externally, causing the displacer to be pushed by the expanding hot air. However this is not a piston, the displacer itself is slightly smaller than the internal bore of the tube, which allows the hot air to pass by it and on through the block to the cold side of the engine. You can see the gap in the last pic.

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[Steven000]

Great machining Bob, looks all very nice and shiny!

 

[Jack L]

I don't really understand anything that's just happened, but it looks i impressive already!!

 

[Airborne01]

Like Jack says - don't understand a tech word but am intrigued, impressed and intimidated by your skill set!
Steve

 

[Miko]

I envy your workshop and tools!!!

I have a Stirling engine powered by a cup of tea!

Miko (I wish I had a mini lathe!)

 

[Jim R]

As others have said the technicalities are beyond me but it's very interesting. It all looks very neatly and skillfully done. Sounds as if you're having fun and developing new skills.

 

[Mr Bowcat]

Many thanks chaps.

If anyone would like me to clarify any of the terms or techniques, feel free to ask.

 

[minitnkr]

I'm experiencing much tool envy. Great work. Murder saw very aptly named. Scary looking!

 

[Mr Bowcat]

Cheers Paul.

Yes, the saw not only looks scary but has a tendency to shatter if pushed too hard. The thought of High Speed Steel shrapnel flying past your head isn't pleasant.

 

[HAWKERHUNTER]

I have no words Bob. I think I need to stop following your builds as they make me feel like an absolute novice. Great works mate.

 

[Mr Bowcat]

Cheers Steve.

 

[Mr Bowcat]

Morning all.

I've been back in the workshop over the last couple of evenings. I decided to get the base and cylinder mounting block made, that way I can add new parts to them as they are made and keep everything together.

The base is just a piece of 60x95x6mm ali plate, with holes drilled for mounting parts.

Then on to the block. This is mild steel, 115x32x11mm. It needed a 5 mm hole all the way through the long section, which is the air passage. Drilling long holes is difficult for several reasons, chiefly keeping the hole straight (twist drills have a tendency to wander) as well as the height limitations of smaller mills. To get around this the hole was drilled from either end and met in the middle, and I am very pleased that it met perfectly so I got something right. To finish the hole each end was tapped M6 for blanking grub screws.

Then it was on to drilling the ports and mounting holes for the displacer and power cylinders, one on each side. The displacer side also needed a brass gland for the displacer rod. This was turned on the lathe to a press fit as again this is the hot side so adhesives won't work. There are also two M4 tapped holes in the bottom of the block for mounting to the base.

With the block complete it just needed two stand offs which were made from 11mm hex bar. Simply drilled through and parted off to length in the lathe. And this is what it looks like so far all bolted together.

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You can start to see how it will work now. The heated air in the displacer tube will expand and flow around the displacer rod and through the port, along the central hole, and then out the other side to the power cylinder port.

 

[Mr Bowcat]

Good morning all.

A very enjoyable weekend was spent in the workshop with several parts completed.

I decided to get the crank assembly done first, this started with the crankshaft supports. They could just be left square and would function perfectly fine, but the drawing called for aesthetic tapered sides and a rounded top, so that's what I did.

Next up was the crank, crank bushings, the power and displacer cranks and their pins.

Then I decided to make the power cylinder. As before this is just a length of seamless tube silver soldered to a flange. This still needs cleaning up and the inside bore polishing before the piston can be made.

Last (and the part which took the most machining) was the flywheel. This started life as a 76x30mm puck of cast iron. It was faced off, then the first 8mm turned down to 20mm to form the boss (that later had a cross drilled M3 tapped hole in order to secure it to the crank). Then the rebate was cut (this is mostly decorative but also partly to reduce the weight) and a 5mm drilled and reamed hole put through the middle. With that side complete a mandrel was turned and tapped in order to re-mount the flywheel the other way around, then it was just a case of removing the excess material (around 8mm) and forming the matching rebate in the other side.

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I really enjoyed turning the cast iron, because it contains around 6-10% graphite it self lubricates and cuts well. However, the mess left behind is biblical! It took a good half hour to clean up the lathe.

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So, this is what it currently looks like. Only a few more parts to make now.

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[Mr Bowcat]

Good morning all.

Getting close to the finish line with this engine. Next part to be made was the piston and the two linkages (the other one is for the displacer). The piston was turned to a close fit with the power cylinder, then the two parts were lapped with some metal polish to a nice sliding fit. Should hopefully get some good compression, and the piston will continue to bed in when the engine runs.

The linkages are just 1.5mm mild steel. Holes were drilled at their respective positions, then the parts were rough milled to close dimension and finished with a file.

So this is how it looks now.

Essentially it would run like this (and does turn over very nicely by hand), but the last part to be made is the cooling fin. This started with some 1.5mm, ali, rough cut and then squared up in the mill to 75mm. 4 x 3mm holes were drilled at 14mm from centre for the securing bolts, and a 5mm hole drilled in the centre. This will be opened up to 10mm, then mounted in the lathe on a mandrel to turn the outside round.